Tube Shields and Heat

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Tron Pesto, Dec 31, 2017.


  1. Tron Pesto

    Tron Pesto Member

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    Do tube shields help dissipate tube heat or do they trap the heat?

    The reason I ask is I'm replacing some shot tube sockets on a 2 x EL84 franken-amp I built years ago. The tubes hang down and they've always run a little warm so I want to kill two birds with one stone: add support AND help dissipate heat.

    So:

    Shielded socket?

    or

    Retainer strap socket?

    What do you think?
     
  2. HotBluePlates

    HotBluePlates Member

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    It's cold out right now where I am. If I'm standing out in the cold, I sure like to have pants on that don't hug my skin as I stand. The little pocket of air between my legs & the pants keeps me warmer than if my legs were in close contact with my pants.

    So it is with tube shields. The kind often used is more for screening of electrical interference, as there is an air pocket between the tube & the body of the shield. Expect the tube will heat that air, and the shield will hold that hot air in & prevent as-effective cooling.

    There are different tube shields that were common in the old days: IERC shields.

    These have an anodized black (sometimes dark purple) outer sleeve that slides over a piece of metal with many small "fingers" which wraps around the tube. There is tight contact from that wrap-wound piece, the tube & the outer sleeve. These dissipate heat quite well. However, if you care about pristine labels on your NOS tubes, be ready... They will absolutely obliterate any printing on a tube due to the tight fit.

    IERC Shields in Use

    [​IMG]

    [​IMG]
     
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  3. Tron Pesto

    Tron Pesto Member

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    Thanks so much for that HBP! That makes sense. I just wasn't sure if maybe since metal conducts heat so much better than air, that the radiant heat the shields picked up was transferred to the chassis faster than the heat was "stored" in the air gap between the tube and the shield.

    So I'm going for retainer wire sockets, thanks again!
     
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  4. Avatar Tech

    Avatar Tech Member

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    Good call IMO. The power tubes should be orders of magnitude less susceptible to stray noise than preamp tubes.
     
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  5. TimmyP

    TimmyP Member

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    I've run across a few amps that required shields to keep them from oscillating at their highest usable gain settings, so it might be wise to at least put the shield bases on so you have the option.
     
  6. bluesky636

    bluesky636 Member

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    I use tube shields on the preamp tubes of all my amps without issues. A 12AX7 gets nowhere near as hot as a 6L6GC or similar power tube.
     
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  7. kinmike

    kinmike Supporting Member

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    My tube shields from my blackface Fenders are in a bag somewhere....no issues.
     
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  8. HaroldBrooks

    HaroldBrooks Member

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    I measured peak (2 hours running) 12AX7 temps with (145 deg F) and without (125 deg F) the shield with an infrared gun. They always get hotter with the shield.

    You could still use the shields if it was designed that way, and employ a small fan to keep things cool.

    A thin coating of flat black paint will increase heat emissivity of ordinary metal quite a bit. I always painted critical sections of my Drag car motors this way, and it kept temps down. http://www.infrared-thermography.com/material-1.htm
     
    Last edited: Jan 27, 2018
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  9. TimmyP

    TimmyP Member

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    How can heat travel from metal to paint to air more effectively than the heat travels from metal to air?
     
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  10. HaroldBrooks

    HaroldBrooks Member

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    The EVEN and THIN sprayed COAT of Flat Black Paint adheres to the metal more or less uniformly, first as a liquid filling the surface imperfections and bonding directly with it, then acting as a highly uneven top surface area at the face that is coating and adhering to the metal below.

    The outward facing emissive surface area of the black paint is larger than the original shiny metallic surface (that's why light is absorbed and it appears black), and therefore absorbs, and it sheds emissive heat at a higher rate.

    If you doubt that last statement, then the same would apply to any radiant heat exchange system. Heat sinks add more surface area to the part they are attached to, via fins that increase the emissive surface area above and beyond the initial surface they are attached to. The original surface is unchanged, but the heat sink cools at a higher rate, allowing the original part to exchange it's heat to the directly connected and cooler heat sink at a higher rate. It's the basis for every radiator made, to increase the area available for heat transfer from one material / surface to another.

    Try it, It works, The trick is to not make the paint too thick, or it acts as an isulator. Just a very thin sprayed coat of flat black will help a bit, not hurt, if the initial surface was shiny metallic to begin with.

    Heat shields are shiny metallic to reflect heat back from coming in, not flat black, and they shouldn't be indirect contact with the part they protect. Shiny Aluminum foil is used to contain heat, or to act as a shield, but not to increase emissivity and liberate heat at a faster rate.

    Increase surface area in direct contact with a part for radiant emissions increase.
     
    Last edited: Jan 29, 2018
  11. HotBluePlates

    HotBluePlates Member

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    Thanks for your test results! This appears to confirm what I said earlier.

    Note that preamp tubes weren't the original question, but EL84 output tubes. Yes, preamp tubes generally do not get nearly as hot as output tubes; they dissipate much less power.

    We might look at a Super Reverb as a worst-case. The input stages show 2.1v across 1.5kΩ cathode resistors (1.4mA of plate current), with the plates at 270v, for ~0.38 watts of plate dissipation. So the 12AX7 is idling at ~38% of its plate dissipation rating. Probably not breaking a sweat, so the heat impact of the shield is minor. FWIW, the heater is sucking 6.3v * 0.3A = 1.89 watts to heat up the cathode & get it to emit electrons.

    We might think the 12AT7 in that Super Reverb represents a more serious situation. The schematic shows 8.4v across a 2.2kΩ cathode resistor (3.82mA of plate current) and ~442v plate-to-cathode, for a plate dissipation of ~1.7 watts. But with a rating of 2.5w, this is ~68% of the plate dissipation rating. It would be hard to know the impact of adding the shield without idling the 12AT7 at max dissipation and getting a bulb temperature, then cooling back down to stock & comparing to IR temps with/without the shield. But long experience with Fender amps suggests the shield isn't much of an issue here.

    But yes, the tube shields can offer shielding from interference, and in some cases, cross-talk among adjacent tubes.

    While true that flat black paint will increase surface area somewhat, the reason for black color is to move towards a better approximation of a black-body radiator to lose heat to surrounding materials/air. Notice those IERC tube shields I mentioned earlier are almost always black; they absorb heat from the tube by direct conduction through metal fingers touching the tube envelope, and radiate heat to the air from the outer shield surface. The black color improves emissivity where it's not practical to meaningfully increase surface area, as you might with a finned heat sink.
     
    Last edited: Jan 28, 2018
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  12. HaroldBrooks

    HaroldBrooks Member

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    As always HotBluePlates, thank you sincerely for offering a better and more intelligent explanation than I came up with !

    I intentionally used the 12AX7 as an example, because it there is a large temperature difference with that tiny low current tube, I would expect it to be that much of a bigger concern for large power tubes.
     
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